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On a Sunday afternoon not long after he was elected vice president, Joe Biden recalls, he learned a lesson about his new station in life.

Biden was finishing up a call with his new chief of staff, Ron Klain, when he dropped the news that his friend Ted Kaufman would be filling his Senate seat.

Joe Biden: Candidate in Pictures

“Did you tell the president?” asked Klain, who, as an aide to Al Gore, had learned something about vice presidents and their place.

“Why the hell should I tell the president?” Biden remembers thinking.

Best of Biden's Gaffes

After all, for 36 years, Biden had been his own man, rising to the Senate’s upper echelons and answering to no one except the voters of Delaware. But Klain’s question caught him up short — a teaching moment Biden would face on several more occasions, and could again — while working to define his role as vice president and tame an oversize personality and a gift for gaffes.

“It dawned on me that Ron was right,” he said in a recent interview with POLITICO. “What if Ted ended up being a jewel thief? It’s not going to be, ‘Well, Biden ... ’ It’s going to be Barack. He’s going to get the blame.”

Team Obama

He called President Barack Obama with the news about Kaufman. But he acknowledges that his instincts are not yet naturally deferential.

“That’s been the hardest part” of adjusting to his new job, said Biden. “I don’t know how to explain it, but I hope I’m getting better at it. Because I’m confident the other part is really working well. I am really satisfied with and actually excited about my ability to be value-added and to be involved in this.”

Asked by POLITICO for an assessment of his vice president, Obama cited Biden’s candor as the attribute he values most.

“The best thing about Joe is that when we get everybody together, he really forces people to think and defend their positions, to look at things from every angle, and that is very valuable for me,” the president said in response to questions submitted by POLITICO. “I also know, when he gives me his advice, he gives it to me straight.”

Biden “is always prepared to be the skunk at the family picnic to make sure we are as intellectually honest as possible,” said one senior aide. And that role has been particularly true as the Obama administration has developed its policy on Afghanistan and Iraq.

As the president has committed more resources to Afghanistan, Biden, former chairman of the Senate Foreign Relations Committee, has been a skeptical voice on a military buildup, sources said. While not staking out a position in absolute opposition, the vice president peppers advisers with questions about the timing and size of any new troop buildup and about the public’s tolerance for it.

As for Iraq, Biden was in Baghdad on Tuesday for the third time as vice president, warning Iraqi leaders that “there are a number of steps that need to be taken” before they can take over security there, even if a referendum is passed calling for a swifter U.S. withdrawal. During his meetings, the Green Zone came under fire, but no one in his party was harmed.

On a visit in July, Biden had a similar warning about the security situation, telling the Iraqis that the U.S. would cut back its engagement in the country if it devolved into sectarian violence.

“He read them the riot act, and he had the most credibility of anybody in the administration to do that,” given his attention to Iraq in the Senate, said Michael O’Hanlon, a foreign policy expert at the Brookings Institution.

Besides making Biden his point man on Iraq and Afghanistan, Obama has also given him the assignment of overseeing the results of the economic stimulus bill and moving his domestic agenda in the Senate — three areas that will help define the Obama presidency.

“I think he has one of the broadest portfolios dealing with the most important issues on the president’s agenda that any vice president has had,” said White House press secretary Robert Gibbs.

Biden attends the president’s intelligence and economic briefings and sessions with Secretary of State Hillary Clinton and Defense Secretary Robert Gates. He also has breakfast with Clinton every Tuesday at his official residence, a few blocks from Clinton’s home. The two former senators develop their agenda, and staff members don’t attend.

Biden and Obama have lunch every week at the White House and have been spotted practicing putts on a White House green. According to Biden, people would be “surprised” at what he and Obama discuss over lunch, especially if they’ve already spent hours together in briefings.

“Half the time, we end up talking about everything from what sports we played, to golf, to my grandchildren and his children,” who attend school together, he said.

When Biden speaks in public and slips, he still elicits a certain amount of eye-rolling and consternation among White House aides. On a visit to Georgia and Ukraine during the summer, Biden described Russia’s economy and population as “withering” in an interview with The Wall Street Journal. The remark irked the Kremlin and left the White House trying to explain what he meant.

“His personality may make him seem too casual, but it also frees him to let it all hang out,” said O’Hanlon. “At times, that works. The challenge for the administration is to find out in advance when it won’t.”

One explanation for his bumpy learning curve, Biden offered, is that he never seriously thought about becoming vice president. He and his wife, Jill, had mulled over the idea of a Cabinet appointment. But not the No. 2 job.

When Obama won the Democratic nomination in the summer of 2008 and first asked if he could vet Biden — and insisted on a quick answer — Biden declined, he said in the interview.

When granted more time to think, Biden asked Klain, a longtime adviser to Biden before becoming Gore’s chief of staff, to prepare memos of the pros and cons of the job — and the pros and cons of Biden being the one to fill it.

“Do vice presidents really matter? Who can tell me who Lincoln’s vice president was? Quick, quick,” he asked his advisers, snapping his fingers.

Biden agreed to be considered after Obama assured him that, as vice president, he’d be consulted on all major decisions and not be relegated to one or two special projects. Biden’s outgoing personality and political instincts make his persistence in pursuing a goal particularly effective. Worried that any reports of waste or corruption in the stimulus program will undermine it with the public, Biden has emerged as the administration’s enforcer. So far it has worked.

“We’re 200 days into the program, and there haven’t been any major headlines saying, ‘The government screwed up,’” he said at a recent Cabinet meeting.

Biden holds weekly stimulus conference calls with mayors and governors to get progress reports and early warning of problems. His staff is required to fix a state’s problem within 24 hours — or inform state officials when it will be.

Tennessee Democratic Gov. Phil Bredesen said an application modification that normally takes months to fix was remedied in five days after he informed Biden’s staff.

“The first meeting — when they were sword-rattling — I know I bridled a little bit, and other governors did, too,” said Bredesen. “But it did help a lot of states to realize they needed to put up solid, well-staffed offices because everyone is looking over our shoulders.”

Elected to the Senate at the age of 29, Biden grew up there, and his relationships are as layered and deep as family — and extremely useful to Obama. Biden is credited with corralling the final votes on the stimulus and budget bills. His staff, teeming with his former Judiciary Committee aides, successfully shepherded Supreme Court Justice Sonya Sotomayor’s nomination.

But the most significant product of his close ties in the chamber came when he talked (and talked and talked) Pennsylvania Sen. Arlen Specter into becoming a Democrat. Since then, he’s quietly helped Specter navigate his new party.

“He’s opened up a lot of doors on meetings with Cabinet officials,” said Specter, noting that three Cabinet secretaries have already headlined campaign events. Biden will appear at an October fundraiser in Pittsburgh.

But the vice president now faces his toughest test: helping ensure that Obama succeeds in getting health care reform through Congress.

Whether Biden can bring Republican or conservative votes along is unknown, but his years of building friendships across party lines will get the White House a hearing in what could be some critical offices.

Sen. Susan Collins (R-Maine), for example, has kept her distance from the bipartisan negotiations on a reform bill. She opposes a government-backed insurance plan but has left herself some flexibility on legislation that doesn’t include it.

No matter how the Senate bill reads, though, she fully expects to be fielding a call from Biden — just as she did in early December when he called to talk about his hopes for bipartisanship. Collins was driving in Northern Maine, and her cell phone kept cutting off.

She expected an aide to call back. But “Joe dialed back every single time,” she said. “The third time, our conversation was really finished, but he called back to say goodbye and Merry Christmas.”

He also called her during debate on the $787 billion stimulus package. Collins voted for it, and Biden called afterward to thank her for the vote.

Another relationship that will be put to the test is with Iowa Sen. Chuck Grassley, the lead Republican health care negotiator and a wild card in the Senate debate.

“I consider Joe Biden a person I’m very comfortable with,” said Grassley, who teamed with Biden as a lowly freshman on a Voting Rights Act reauthorization more than two decades ago. “It’s been that way since my first year on the Judiciary Committee in 1981.”